This invention relates to formation of polymeric particles of narrow size variation within the range having particle size between about 5 microns to about 50 microns by a suspension polymerization process.
The formation of small polymeric particles for use in electrostatic powder coating, fluidized bed coating and plastisols has generally been carried out by processes such as emulsion polymerization and dispersion of polymer in a liquid heated to above its melting temperature and then cooling in the liquid to form spherical particles. Using these processes, it has been difficult to achieve particles of small spherical particle size and narrow size range. The particles formed by conventional suspension polymerization are larger than desirable for powder coating or use in plastisols. Grinding or attrition, especially fluid energy milling, of larger particles to the size needed for powder coating is often not desirable both from an economic and functional viewpoint. Electrostatic powder coating with smaller particles is desirable as it allows complete coating of the article with a thinner film than is possible if only large particles are used. In order to obtain narrow ranges of small particles, it has previously been necessary to classify particles of a wide size range to separate the small ones with the resultant expense. Further, processes such as spray drying of polymer suspended in solvent can result in polymeric particles of a wide size range, as well as trapping of solvent which interferes with the use of the particles in plastisols or electrostatic powder coatings.
U.S. Pat. No. 3,505,434 discloses a process wherein particles suitable for fluidized bed coating are prepared by dispersing the polymer in a liquid which is heated to about 20.degree. C above the polymer melting point and stirred, causing the polymer particles to attain a spherical or nearly spherical shape. The particles are then cooled below their melting point and recovered. However, this process apparently does not produce particles of narrow size distribution or of a size which is below 50 microns.
Suspension polymerization of monomer is a well known process for formation of polymer particles generally in a size range of about 200 to 600 microns. The advantages of suspension polymerization is that the configuration of the product is a bead or sphere which may easily be recovered and further that the dissipation of heat of formation is facilitated by the suspending phase. It is difficult by suspension polymerization to make small particles as the particles tend to coalesce during the polymerization process.
U.S. Pat. No. 3,819,597 discloses a process of suspension polymerization for producing large particle sizes using a two-stage process in which the monomer is partially polymerized prior to suspension in water for completion of polymerization.
U.S. Pat. No. 3,243,419 discloses a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in prevention of the coalescence of the particles. It is noted that in neither of these patents, is the size distribution narrow or the size small.
As can be seen, there remains a need for a process of producing polymer particles of spherical shape and narrow size range for average sizes below about 50 microns. Further, there remains a need for a process which would produce small particles without numerous polymer forming steps followed by particle forming steps. Further, there remains a need for the production of colored polymeric particles for use in electrostatic powder coating, fluidized bed coating and formation of plastisols.